Light diffusing film

ABSTRACT

A light diffusing film which is excellent in luminance uniformity and which exhibits no partial deterioration in light transmittance, thereby eliminating the formation of black spots. The light diffusing film, which has a monodisperse light diffuser and a light diffusing layer consisting of a light diffuser and a binder resin on one plane of a base, in which the lamination ratio is 10% or less and the diameter of flocculated particles in the light diffuser is 50 μm or less, exhibits excellent luminance and thermal properties, so that it can be suitably used in various kinds of picture display devices, in particular, in a display device employing LED (light emitting diode) lamps or an LCD (liquid crystal display device) backlight system.

TECHNICAL FIELD

The present invention relates to a light diffusing film, and moreparticularly, to a light diffusing film which is excellent in luminanceuniformity and which exhibits no partial deterioration in lighttransmittance, thereby eliminating the formation of black spots.

BACKGROUND ART

In industry, polymeric resin films have a wide variety of applicationsincluding bases for packing, photographic films, condensers, electricinsulators, labels, magnetic recording media, medical purposes. Inrecent years, polymeric resin films have also been in widespread use asbase materials for picture display devices such as rear projection typescreens, plasma display panels or liquid crystal display devices.

In particular, since a picture display device, unlike a cathode raytube, adopts an indirect illumination system in which light irradiatedfrom a fluorescent tube of a backlight provided on the backside of thedevice is reflected to a metal deposition layer or semi-transparent oropaque, white reflecting plate to display a picture through a lightguide plate, visibility is poor, which becomes worsened according topursuit of larger display devices.

To overcome the above-described problem, a light diffusing film that isadhered to a light guide plate to increase luminance by uniformlydiffusing light, has been developed.

For example, Japanese laid-open patent Nos. hei 6-67003, 7-174909 and9-127314, U.S. Pat. No. 5,706,134 and Japanese Utility Model No.2,539,491 disclose light diffusing films having a transparent binderresin solution in which inorganic or organic particles are dispersed,coated on one surface of a transparent polymeric resin film as a lightdiffuser.

Here, either an inorganic light diffuser or an organic light diffusermay be used as the light diffuser of the present invention. Examples ofthe inorganic light diffuser include silica, zirconia, calciumcarbonate, barium sulfate, glass, titanium dioxide, and so on, andexamples of the organic light diffuser include fine particles ofurethane based resins, acryl based resins, vinyl chloride based resins,and so on.

The diameter of the light diffuser is typically in the range of 1 to 50μm. In order to maximize the formed of a poly of the light diffuser perunit area, particles having different particle sizes are generally used.

Also, in order to increase diffusion transmittance, there has been anattempt to maximize the ratio of a light diffuser to a transparentbinder resin. However, in such a light diffusing layer, light diffusingmaterials cannot exist independently but flocculate with one another orexist in the form of laminates.

FIG. 3 is a cross-sectional view of a conventional light diffusing film.The transmittance of light transmitted through a light guide plate P islowered at a portion L where light diffuser particles S are laminated.In particular, the transmittance is noticeably reduced at a portionwhere the particles are flocculated. Thus, there is a limit in improvingthe uniformity in luminance. Also, black spots are easily visible bynaked eye, which makes the conventional light diffusing film unsuitableas a light diffusing film.

DISCLOSURE OF THE INVENTION

To solve the above-described problems, it is an object of the presentinvention to provide a light diffusing film having excellent luminanceuniformity while exhibiting no partial deterioration in lighttransmittance and generating no black spots.

To accomplish the above object, there is provided a light diffusing filmhaving, on at least one surface of a base film, a light diffusing layermade of an inorganic or organic light diffuser and a binder resin,wherein the lamination ratio is represented by the formula 1 and thesize of flocculated particles in the laminated light diffuser isrepresented by the formula 2:

$\begin{matrix}{{\text{Lamination~~ratio} = {{\frac{\pi \times {S\left( {µ\; m} \right)} \times \text{Number~~of~~laminated~~particles}}{480\mspace{14mu} µ\; m \times 480\mspace{20mu} µ\; m} \times 100(\%)} \leq {10\%}}},} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$Diameter of flocculated particles=S(μm)×Number of flocculatedparticles≦50μm  [Formula 2]wherein S denotes the average particle diameter of the light diffuser.

The light diffuser is preferably made of one or more kinds ofmonodisperse particles. If two kinds of monodisperse particles are mixedin the light diffuser, the following formula 3 are satisfied:Sa^(1/2)<Sb<Sa  [Formula 3]wherein Sa denotes the average particle diameter of a relatively largerlight diffuser particle and Sb denotes the average particle diameter ofa relatively smaller diffuser particle. If two kinds of monodisperseparticles are mixed in the light diffuser, the coating amount ispreferably in the range of 5 to 25 to 25 g/m². Also, an average particlediameter of the light diffuser is in the range of 1 to 50 μm.

BEST MODE FOR CARRYING OUT THE INVENTION

In a light diffusing film D of the present invention, having a lightdiffusing layer C consisting of an inorganic or organic light diffuser Sand a binder resin B on one plane of a base F made of a polymeric resinfilm, the light diffuser is one of the most influential factors indetermining the luminance of the light diffusing layer according to itsdiffused state, kind and particle size. Particularly, the diffused stateis the most important factor in determining the luminance of the lightdiffusing layer. If the light diffuser is diffused nonuniformly orflocculated, the lamination ratio may exceed 10% resulting in aconsiderable reduction in light transmittance due to flocculatedparticles.

Even if the lamination ratio is 10% or less, flocculation of laminatedlight diffuser particles of greater than 50 μm in size may cause areduction in light transmittance.

There are various techniques of adjusting the lamination ratio to be 10%or less and the size of flocculated particles to be 50 μm or less.

Referring to FIG. 1, which is a cross-sectional view of a lightdiffusing film according to an embodiment of the present invention, thelight diffusing film is formed of a light diffusing layer satisfying theformulas 1 and 2.

In this case, a single-layered light diffusing layer can be obtained bycoating the light diffuser to a uniform thickness corresponding to theparticle diameter, and the light diffuser has preferably monodisperseform.

In order to maximize the luminance of the light diffuser by increasingthe formed of a poly of the same in a desirable range of the presentinvention, two kinds of light diffusers which are different in averageparticle size are preferably used in combination so as to satisfy theformula 3:Sa^(1/2)<Sb<Sa  [Formula 3]wherein Sa denotes the average particle diameter of a relatively largerlight diffuser particle and Sb denotes the average particle diameter ofa relatively smaller diffuser particle.

Preferably, the mixture ratio of particles having an average particlediameter of Sa to particles having an average particle diameter of Sb isin the range of 10:90 to 90:10.

FIG. 2 is a conceptual diagram of a light diffusing film according toanother embodiment of the present invention. Referring to FIG. 2, alight diffusing layer satisfying the formulas 1, 2 and 3 is formed ofanother monodisperse particles.

Here, an average particle size of the light diffuser is preferably inthe range of 1 to 50 μm. If the average particle size of the lightdiffuser is less than 1 μm, monodispersion is difficult to achieve,which makes light diffuser particles prone to flocculation. Also, thelamination ratio may exceed 10%. If the average particle size of thelight diffuser is greater than 50 μm, monodispersion is easily achievedbut interparticle attraction becomes strong so that flocculation islikely to occur again after monodispersion. Also, the lighttransmittance may be reduced.

In the case of using light diffusers having different particle diameterssatisfying the formulas 3, the coating amount is preferably in the rangeof 5 to 25 g/m². If the coating amount is less than 5 g/m², which makesthe film too thin, the light diffusing effect is negligible. If thecoating amount is greater than 25 g/m², the lamination ratio exceeds10%, resulting in a reduction of light transmittance, which isdisadvantageous from the viewpoint of practicality as a light diffusingplate.

The light diffusing film satisfying the above-described requirements canbe manufactured by general film making methods.

First, a light diffusing composition containing a light diffuser and abinder resin is dispersed using a general disperser such as a beadmiller using glass or zirconium beads or a roll miller using horizontalor vertical rolls.

In order to promote dispersion, a diffuser containing denaturedpolysiloxane or polycarboxylic acid may be used.

The binder resin solution having light diffusers uniformly dispersed, iscoated on a base film, preferably a PET film, and dried to thus form alight diffusion layer.

Methods of coating a light diffusing composition on a base film includemethods generally known in the art without limitation, and usablecoating methods include air knife coating, gravure coating, reverse rollcoating, spray coating and blade coating.

After the base film is coated with the light diffusing composition, theresultant structure is heated by irradiating hot air, infrared rays orfar infrared rays, thereby forming a coating layer.

The term “monodisperse light diffuser” used throughout the specificationmeans a light diffuser in which approximately 95% of the light diffuserparticles are within the range of ±15% in terms of an average particlesize of the light diffuser.

The present invention will now be described more fully with reference tothe following examples. The invention may, however, be embodied indifferent forms and should not be construed as limited to the examplesset forth herein. Rather, these examples are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope of the invention to those skilled in the art.

EXAMPLE 1

A light diffusing composition having the following constituents wascoated on a 100 μm thick PET film (Model No. SH31 of SKC Co., Ltd.,Korea) at a dose of 15 g/m² using a 0.3 mm myer bar and dried at 120° C.for 3 minutes, thereby manufacturing a light diffusing film having alamination ratio of 1.2% and a flocculated particle diameter of 20 μm.Then, the black spots and luminance characteristic of the manufacturedlight diffusing film were evaluated, and the results thereof are listedin Table 1.

Binder resin 25.4 parts by weight (Nipporan 125, Nippon PolyurethaneCo.) Curing agent 2.9 parts by weight (Coronate HX, Nippon PolyurethaneCo.) Monodisperse polymethylmethacrylate light diffuser 30.3 parts byweight (MX-1000 having an average particle diameter of 10 μm, SokenChemical Co., Ltd.) Solvent 41.4 parts by weight (MEK:toluene = 1:1)

EXAMPLE 2

A light diffusing film having a lamination ratio of 2.7% and aflocculated particle diameter of 25 μm was prepared from the followingcomposition by the same equipment and method as in Example 1. Then, theblack spots and luminance characteristic of the prepared light diffusingfilm were evaluated, and the results thereof are listed in Table 1.

Binder resin 25.4 parts by weight (Nipporan 125, Nippon PolyurethaneCo.) Curing agent 2.9 parts by weight (Coronate HX, Nippon PolyurethaneCo.) Monodisperse polymethylmethacrylate light diffuser 21.2 parts byweight (MX-1000 having an average particle diameter of 10 μm, SokenChemical Co., Ltd.) Monodisperse polymethylmethacrylate light diffuser21.2 parts by weight (MX-500 having an average particle diameter of 5μm, Soken Chemical Co., Ltd.) Solvent 41.4 parts by weight (MEK:toluene= 1:1)

EXAMPLE 3

A light diffusing film having a lamination ratio of 0.8% and aflocculated particle diameter of 30 μm was prepared from the followingcomposition by the same equipment and method as in Example 1. Then, theblack spots and luminance characteristic of the prepared light diffusingfilm were evaluated, and the results thereof are listed in Table 1.

Binder resin 25.4 parts by weight (Nipporan 125, Nippon PolyurethaneCo.) Curing agent 2.9 parts by weight (Coronate HX, Nippon PolyurethaneCo.) Monodisperse polymethylmethacrylate light diffuser 30.3 parts byweight (MX-1500 having an average particle diameter of 15 μm, SokenChemical Co., Ltd.) Solvent 41.4 parts by weight (MEK:toluene = 1:1)

COMPARATIVE EXAMPLE 1

A light diffusing film having a lamination ratio of 30.2% and aflocculated particle diameter of 90 μm was prepared from the followingcomposition by the same equipment and method as in Example 1. Then, theblack spots and luminance characteristic of the prepared light diffusingfilm were evaluated, and the results thereof are listed in Table 1.

Binder resin 25.4 parts by weight (Nipporan 125, Nippon PolyurethaneCo.) Curing agent 2.9 parts by weight (Coronate HX, Nippon PolyurethaneCo.) Monodisperse polymethylmethacrylate light diffuser 30.3 parts byweight (Epostart MA-1010 having an average particle diameter of 0.03 to26 μm, Japan Catalyst Co., Ltd.) Solvent 41.4 parts by weight(MEK:toluene = 1:1)

COMPARATIVE EXAMPLE 2

A light diffusing film having a lamination ratio of 21.5% and aflocculated particle diameter of 81 μm was prepared from the followingcomposition by the same equipment and method as in Example 1. Then, theblack spots and luminance characteristic of the prepared light diffusingfilm were evaluated, and the results thereof are listed in Table 1.

Binder resin 25.4 parts by weight (Nipporan 125, Nippon PolyurethaneCo.) Curing agent 2.9 parts by weight (Coronate HX, Nippon PolyurethaneCo.) Monodisperse polymethylmethacrylate light diffuser 21.2 parts byweight (MX-1500 having an average particle diameter of 15 μm, SokenChemical Co., Ltd.) Monodisperse polymethylmethacrylate light diffuser9.1 parts by weight (MX-150 having an average particle diameter of 1.5μm, Soken Chemical Co., Ltd.) Solvent 41.4 parts by weight (MEK:toluene= 1:1)

<Evaluation>

Lamination Ratio

Two or three sheets of light diffusing films were laminated on a lightguide plate of a liquid crystal backlighting unit and then observedusing an optical microscope at a 500 times magnification to calculatethe lamination ratio represented by the formula 1.

Maximum Diameter of Flocculated Particles

Two or three sheets of light diffusing films were laminated on a lightguide plate of a liquid crystal backlighting unit and then observedusing an optical microscope at a 500 times magnification to calculatethe particle diameter represented by the formula 2.

Black Spots

Two or three sheets of light diffusing films were laminated on a lightguide plate of a liquid crystal backlighting unit and then observedusing an optical microscope at a 14 times magnification to identifyblack spots by naked eye. The quality (goodness or badness) of the filmswas rated as having one of the following grades:

good: neither dots nor black spots caused by lamination were found;

poor: dots or black spots caused by lamination were found.

Luminance

Two or three sheets of light diffusing films were laminated on a lightguide plate of a liquid crystal backlighting unit and then the luminancewas measured using a luminance meter BM-7 available from TOPCON Co.,Ltd.

TABLE 1 Compar- Compar- Example Example Example ative ative 1 2 3Example 1 Example 2 Sa (μm) 10 10 12 10 15 Sb (μm) — 5 — — 1.5 Ratio of10:0 7:3 10:0 10:0 7:3 Sa to Sb used Lami- 1.2 2.7 0.8 30.2 21.5 nationratio (%) Diameter 20 25 30 90 81 of flocculated particles (μm) Blackspot Good Good Good Poor Poor characteristic Luminance 1723 1720 17251683 1694 (Cd/m²)

As shown in Table 1, in Examples 1 through 3 in which the laminationratio is 10% or less and the size of flocculated particles in the lightdiffuser is 50 μm or less, the luminance characteristics are excellentand no black spots are found.

However, in Comparative Examples 1 and 2 in which flocculation andlamination occur to the light diffusing films, the luminancecharacteristics are poor and black spots are found, which aredisadvantageous in view of applicability for various kinds of displaydevices.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

INDUSTRIAL APPLICABILITY

The light diffusing film according to the present invention, which has amonodisperse light diffuser and a light diffusing layer consisting of alight diffuser and a binder resin on one plane of a base, in which thelamination ratio is 10% or less and the diameter of flocculatedparticles in the light diffuser is 50 μm or less, exhibits excellentluminance and thermal properties, so that it can be suitably used invarious kinds of picture display devices, in particular, in a displaydevice employing LED (light emitting diode) lamps or an LCD (liquidcrystal display device) backlighting system.

1. A light diffusing film having, on at least one surface of a basefilm, a light diffusing layer made of an inorganic light diffuserselected from the group consisting of silica, zirconia, calciumcarbonate, barium sulfate, glass and titanium dioxide; or organic lightdiffuser selected from the group consisting of urethane based resins,acryl based resins and vinyl chloride based resins, and a binder resin,wherein a lamination ratio is represented by the Formula 1 and a size offlocculated particles in a laminated light diffuser is represented bythe Formula 2, wherein the light diffuser is made of two or more kindsof monodisperse particles, the following Formula 3 is satisfied, whereinan average particle diameter of the light diffuser is in the range of 5to 50 μm: $\begin{matrix}{{\text{Lamination~~ratio} = {{\frac{\begin{matrix}{\pi \times {S\left( {µ\; m} \right)} \times {Number}\mspace{14mu}{of}\mspace{14mu}{laminated}\mspace{14mu}{particles}} \\{{in}\mspace{14mu} a\mspace{14mu}{tested}\mspace{14mu}{portion}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{light}\mspace{14mu}{diffuser}}\end{matrix}}{480\mspace{14mu} µ\; m \times 480\mspace{14mu} µ\; m} \times 100(\%)} \leq {10\%}}},} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$Diameter of flocculated particles=S(μm)×Number of flocculated particlesin a tested floc and in a tested portion of the lightdiffuser≦50μm  [Formula 2] wherein S denotes the average particlediameter of the light diffuser,Sa^(1/2)<Sb<Sa  [Formula 3] wherein Sa denotes an average particlediameter of a relatively larger light diffuser particle and Sb denotesan average particle diameter of a relatively smaller diffuser particle.2. The light diffusing film according to claim 1, wherein a coatingamount of the light diffuser is in the range of 5 to 25 g/m².
 3. Adisplay device employing LED (light emitting diode) lamps or an LCD(liquid crystal display) backlight system, and comprising the lightdiffusing film according to claim
 2. 4. A display device employing LED(light emitting diode) lamps or an LCD (liquid crystal display)backlight system, and comprising the light diffusing film according toclaim 1.